TTF-DDQ: Two “green” synthetic routes, crystal structure and band gap from FT-IR spectroscopy

• Mechanochemical, vapor digestion and solution synthesis of TTF-DDQ.
• Crystal structure analysis of TTF-DDQ from synchrotron X-ray powder diffraction.
• Band gap estimation from the FT-IR absorption edge.

TTF-DDQ (DDQ = 2,3-dichloro-5,6-dicyano-p-benzoquinone) is an ionic and diamagnetic charge transfer salt. Polycrystalline powders of TTF-DDQ were obtained by three synthetic methods: solvent evaporation from MeCN solution (1), MeCN vapor digestion (2), and mechanochemistry (liquid assisted grinding with DMSO) (3). The analysis of the synchrotron X-ray powder diffraction data of 1 afforded the crystal structure, which is isomorphous to that of the black polymorph of TTF-CA (CA = chloranil). TTF-DDQ is composed of (TTF+)2 and (DDQ−)2 dimeric radical ions arranged in segregated columnar stacks, linked by weak hydrogen bonds. The Cl and CN substituents in DDQ are disordered, and this was modeled by Rietveld analysis.The FT-IR absorption spectra as a function of the temperature in the 300–10 K interval showed an absorption edge, indicating that TTF-DDQ is a semiconductor with a small band gap of (0.21 ± 0.01) eV. The monotonous evolution of the FT-IR spectra denotes the absence of crystallographic phase transitions in the temperature interval studied, likely due to the thermodynamic stability of this packing motif compared to the less stable donor-acceptor alternating stacks; or the segregated, potentially conducting stacks.

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